Honeycomb structures are used, for example, as a catalyst carrier for an internal combustion engine, a boiler, a chemical reactor, a reformer of fuel cell or the like, or as a filter for particulate matter present in an exhaust gas, particularly diesel particulate matter.
In the honeycomb structure used for such a purpose, the sharp temperature change of exhaust gas and the local heating make non-uniform the temperature distribution inside the honeycomb structure, which has caused problems such as crack generation in honeycomb structure and the like.
When the honeycomb structure is used particularly as a filter for trapping a particulate matter in an exhaust gas emitted from a diesel engine (the filter is hereinafter referred to as DPF), it is necessary to burn the carbon particles trapped on the filter to remove the particles and regenerate the filter and, in that case, high temperatures are inevitably generated locally in the filter; as a result, a big thermal stress and cracks have tended to generate.
Hence, it was proposed to bond a plurality of honeycomb segments with a cement to produce a honeycomb structure.
For example, in U.S. Pat. No. 4,335,783 is disclosed a process for producing a honeycomb structure, which comprises bonding a number of honeycomb parts using a discontinuous cement.
Also in JP-B-61-51240 is proposed a heat shock-resistant rotary regenerating heat exchanging method which comprises forming, by extrusion, matrix segments of honeycomb structure made of a ceramic material; firing them; making smooth, by processing, the outer peripheral portions of the fired segments; coating the to-be-bonded areas of the resulting segments with a ceramic cement having, when fired, substantially the same mineral composition as the matrix segments and showing a difference in thermal expansion coefficient, of 0.1% or less at 800 degree C.; and firing the coated segments.
Also in a SAE article 860008 of 1986 is disclosed a ceramic honeycomb structure obtained by bonding cordierite honeycomb segments with a cordierite cement.
These honeycomb structures, however, are not sufficient in bonding strength between a honeycomb segment and a bonding layer. Further the honeycomb structures have bonding defects such as a separation or a crack at the interface between a bonding layer and a honeycomb segment caused by differences in thermal expansion coefficient, drying shrinkage, and the like between the bonding layer and the honeycomb structure.
In order to solve these problems, in JP-A-8-28246 is disclosed a ceramic honeycomb structure obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles. However, use of the organic binder (which is difficult to handle) makes long the drying time of sealant, makes difficult the homogenization of sealant composition and makes more the number of materials used in sealant; as a result, a lower productivity is invited.
Hence, there is desired a honeycomb structure which is small in bonding defects and has a high bonding strength, even though it contains no organic binder.